A linear carrier-frequency amplifier (e.g., a radio-frequency (RF) amplifier) is an amplifier that increases the power of a modulated carrier signal while preserving both the amplitude and phase modulation components of the signal. Whereas, a saturating RF amplifier, commonly used in freqency-modulation (FM) radio transmitters, perserves phase modulation of the signal, but may remove or severely distort any amplitude modulation (AM) component. FM signals typically have constant amplitude, and thus, do not require linear amplification. On the other hand, future digital transmission systems may require both amplitude and phase modulation (PM) of the transmitted signals. Linear amplifiers are therefore necessary to transmit those signals.
Linear RF amplifiers commonly use feedback to control the output power. For stability (i.e., freedom from oscillation) a feedback loop must have a gain/phase versus frequency characteristic such that the open loop gain (i.e., the gain of the entire feedback loop with the loop broken at one point) is less than one at all frequencies where the phase shift around the loop is more than 180 degrees different from that at midband. Previous RF feedback loops have used high Q (i.e., the merit factor of the loop) resonant circuits at the carrier-frequency to achieve this condition. This approach has several disadvantages, including large size, a large required RF gain, and the limited amount of open loop gain possible.